Low-pressure mercury vapor discharge lamp



Oct. 31, 1961 A. LOMPE EI'AL 3,007,071

LOW-PRESSURE MERCURY VAPOR DISCHARGE LAMP Filed April 23. 1959 lnvan tor's: Arved Lompe fierber t D we Their Ad: orney.

United States I Patent 3,007,071 LOW-PRESSURE MERCURY VAPOR DISCHARGE LAMP Arved Lompe, Berlin-Zehlendorf, and Herbert Dziergwa,

Berlin-Nikolassee, Germany, assignors to Patent-Trenhand-Gesellschaft fur elektrische Gluhlampen m.b.H.

Filed Apr. 23, 1959, Ser. No. 808,334 Claims priority, application Germany Apr; 29, 1958 Claims. (Cl. 313-178) This invention relates to low-pressure mercury vapor discharge lamps, more particularly to fluorescent tubes having relatively high electrical or thermal loading.

Such tubes contain besides mercury a rare gas, generally argon at a low pressure to initiate starting. The light output of such tubes does not depend only on the electrical wattage or loading applied to the lamp but also on the efliciency of excitation of mercury resonance radiation which excites in turn the fluorescent material. This mercury resonance radiation is at a maximum at a cert-ain constant and relatively low mercury vapor pressure of a few microns. With increasing power input into the lamp, however, the temperature and therefore the temperature-dependent mercury vapor pressure increases because of the higher energy conversion in the discharge. The specific emission of resonance radiation decreases, and consequently the luminous flux does not increase proportionally to the applied input. The decrease in specific light output in fluorescent lamps with increased loading, and the resulting lower efficiency made such lamps unacceptable in the past.

More recently the efliciency of highly loaded fluorescent lamps has been increased to the point where they may be operated with double the formerly conventional loading, or even more. This may be obtained, for instance, by multiple grooving of the tubular bulb so that the lamp surface and, thereby, the heat conduction of the tube are increased. A drawback to this arrangement is the greater cost of manufacture of the lamp envelope. Another way of obtaining better light output with higher loading consists in providing cooler zones at the ends of discharge lamps by shielding the ends against the heat of the electrodes and of the discharge. By maintaining the ends at about 40 0., some part of the mercury condenses and the vapor pressure in the discharge is regulated. However this produces unsightly dark spaces at the lamp ends.

This invention discloses a new means of increasing the efficiency of highly loaded low-pressure mercury vapor discharge lamps, more particularly fluorescent lamps wherein the operating voltage amounts to less than of the supply voltage, by utilizing the fact that the vapor pressure above an alloy is lower than above a pure metal. Lamps made according to'the present invention are dis tinguished by the fact that there isprovided at least one amalgam at places exposed to temperatures substantially 3,007,071 Patented Oct. 31, 1961 ice ' with undersaturated mercury vapor and without addi- 7 hand, attain a balanced condition after a short time.

tional reactance and active resistance, the object of the invention could not be realized because lamps having these features were operated at low current intensities only.

In yet other lamps of the high pressure mercury vapor kind, by the addition of rubidium, cadmium and zinc color improvement has been obtained through co-excitation of the added element. By contrast, in lamps according to the present invention the cadmium is not excited to radia tion; in other words, the loading and therefore the vapor pressure are sufliciently 'low that substantially only the mercury is excitedto radiation. High-pressure discharge lamps operate with pressures and temperatures several orders of magnitude higher and, consequently, faroutside the range required for the optimum of 2537 A radiation, so that the conditions in these lamps cannot be compared with those in low pressure discharge lamps.

In the literature there are to be found only such statements referring to properties of amalgams in liquid condition at higher temperatures. With the present invention the prejudice that amalgams have the required reaction speed at higher temperatures only had to be overcome. It came as a surprise on using the present invention that, by suitable selection and composition of the amalgam, the required reaction speed in respect of change of pressure can be obtained also at temperatures prevailing in lowpressure discharge lamps.

According to the present invention, those metals have proved very suitable which, on the one hand, amalgamate in .a good manner with the mercury and, on the other It is, however, well known that with most of the metals a balanced condition is obtained only very slowly at room temperature. Now, there have been found as suitable for the present invention several metals with which this condition may be obtained in a shorter time. A particularly 0 therewith is smaller than 1, e.g. thallium, cadmium,

not higher than those of the discharge whereby the optimum of efliciency of the mercury resonance radiation is shifted toward higher lamp temperatures by comparison with lamps without amalgam-forming metals.

Amalgam-forming metals have been used in discharge lamps in the past. In low-pressure discharge lamps the amalgam was, for instance, applied to the electrodes or near the electrodes. In these lamps, the amalgam was strongly heated by the electrodes and produced a higher vapor pressure than the mercury in the discharge at a much lower temperature, so that the mercury vaporized continuously out of the amalgam until it was completely decomposed. When the lamp was extinguished, the amalgam-forming metal localized the mercury on the electrode or near the electrode in order to have again a sufliciently high mercury vapor pressure in the lamp when indium, gallium etc. because then the quantity of metal to be added to the mercury is not too great. The activity coefficient with which the'mole fraction must be multiplied in order to obtain the activity, that is the proportion of the vapor pressure above the mixed phase to the vapor pressure of the pure metal at equal temperature, indicates to the extent that it differs from 1 that the behavior of a solution or of an alloy is not ideal. According to the present invention an atomic proportion, if for as a fine-grained powder or film, on the luminescent coating, or as a translucent pnotective layer or in the form of an igniting strip or on an underlayer. As previously mentioned, it is located at predetermined places where,

it is not exposed to temperatures much higher than those in the discharge. Consequently, it is a feature of the present invention that the amalgam does not die directly near or on the electrodes.

Where the amalgam is to be used in powder form, it is desirable to add in the grinding process a hard, neutral material such as silica Si because the amalgams are, generally, soft and smear when pulverized. The amalgam may be introduced into the finished lamp. If it is introduced before the lamp is finished, it may be desirable to provide for free mercury beside the amalgam in order to have the desired composition of amalgam after the heating process. There may also be brought into the discharge tube simultaneously several amalgamforming metals, e.g. cadmium and gold.

Another feature according to the present invention con-. sists in that the amalgam or one of its components is intimately mixed or alloyed with a low melting metal, e.g. gallium, soft solder or the like. The mixture is brought into the finished lamp'to a predetermined place and heated; after this heat treatment the amalgam ad'-, heres fast to the base layer so that it is, in any case,- prevented from coming near the electrodes.

The filling pressure of starting gas in lamps according to the present invention should, suitably, be chosen somewhat lower than in conventional lamps. It may also be advantageous to use instead of argon another rare gas, e.g. neon, either simultaneously or alone as the base gas.

In the accompanying drawing, FIGS. 1 and 2 illustrate by way of examples fluorescent lamps embodying the invention.

The illustrated lamps are designed for operation on a line voltage of 220 volts. The tubular bulb 1 to the inner wall of which the luminescent coating 2 is applied has a diameter of 38 mm. and a length of 1.2 mm., being similar in dimensions to the bulb of an ordinary 40 watt fluorescent lamp. There are used as electrodes 3 the well known oxide pasted coils. In the lamp of FIG. 1, the amalgam-forming metal 4 is, for instance, cadmium which is distributed as a fine-grained powder lying in amalgamated condition on the luminescent coating. The starting gas is a filling of argon at a pressure of '1 mm.

In the lamp of FIG. 2 which is otherwise the same as that of FIG. 1, the amalgam of cadmium is provided as a starting strip or narrow conductive ribbon 5 extending substantially the length of the lamp on its internal surface but stopping short of the electrodes.

Electrical data for lamps similar to the foregoing with mercury alone and with cadmium amalgam in different atomic proportions are given in the following table for inputs of 80 watts and 100 watts:

Input: 80 watts Opratin'gvoltage, volts Lamp current, amps 84 l. 26 Efficiency, 1m./w

Input: 100 watts Operating voltage, volts- Lamp current, amps... Efliciency, lm./W a.

It can be seen from'the above table that for lamps operated with an input of 100 watts, an atomic proportion of CdzHg of 211 is preferable from the point of View of elficiency. For lamps operated with an input of 80 watts, a proportion of Cd:Hg of 1:1 is preferable. It can also be seen that the gain in efficiency due to the addition of the amalgamating metal is higher the more the lamp is loaded. Consequently when an amalgam is used, fluorescent lamps may be operated at higher temperatures and such lamps have a higher efficiency than ordinary fluorescent lamps of equal outer dimensions. By the addition of amalgamating metal the pressure of mercury vapor at the higher operating temperatures is reduced to a value which guarantees optimum output of mercury resonance radiation.

The efliciency of lamps according to the present invention is increased 10 to 15% by comparison with generally used 40 watt lamps, so that the gain in light flux is considerable. Thus the field of use of fluorescent lamps, for instance in industry, commerce and street lighting, is 0011 siderably extended by highly loaded lamps according to the present invention. The invention may also be used in lamps in which the mercury radiation is utilized directly, that is without excitation of luminescent material, as in germicidal lamps.

A further advantage of lamps according to the present invention consists in that the range of possible temperature shifting is much wider than may be obtained by means of the cooling arrangements known hitherto.

The specific embodiments of the invention which have been described are given by way of illustrative example of the invention whose scope is to be determined by the appended claims.

What we claim as new and desire to secure by Letters Patent of the United States is:

l. A low pressure mercury vapor electric discharge lamp wherein the operating voltage does not exceed two thirds of the supply voltage, comprising an elongated envelope having a pair of electrodes sealed into opposite ends and containing a filling of mercury vapor, and an amalgam of mercury with a metal located at places within said envelope exposed to temperatures not substantially higher than those of the discharge therethrough, the metal of said amalgam being selected to shift upward the optimum temperature for generation of mercury resonance radiation within said lamp, the loading of said lamp being sufiiciently low that substantially only the mercury is excited to radiation.

2. A lamp as defined in claim 1 wherein the metal of said amalgam .is selected from the group consisting of thallium, cadmium, indium, gallium and alloys thereof.

3. A lamp as defined in claim 1 wherein said amalgam is provided as a fine-grained powder within the lamp.

4. A lamp as defined in claim 1 wherein said amalgam is provided in the form of an igniting strip extending internally through the lamp.

5. .A lamp as defined in claim 1 wherein said amalgam is admixed with a hard,'neutral material.

6. A lamp as defined in claim 1 wherein said amalgam is admixed with silica.

7. A lamp as defined in claim 1 wherein said amalgam is provided as a fine-grained powder admixed with silica and extending substantially throughout the lamp over a large area.

8. A lampas-defined in claim 1, wherein said amalgam is mixed or alloyed with a low melting metal.

9. A low pressure mercury vapor electric discharge lamp comprising an elongated vitreous envelope having a pair of electrodes sealed into opposite ends and containing a filling of an inert starting gas at a low pressure and mercury vapor at a pressure of a few microns, and an amalgam of mercury with cadmium located at places within said envelope exposed to temperatures not substantially higher than those of the discharge therethrough whereby to shift upward the temperature for optimum mercury vapor pressure corresponding to maximum mercury resonance radiation within said lamp, the loading of said lamp being sufficiently low that substantially only the mercury is excited to radiation. Y

10. A lamp as defined in claim 9 wherein the atomic 2,228,327 proportion of cadmium relative to mercury in said amal- 2,291,965 gain is in the range of 0.5 to 5 relative to 1. 2,476,681

References Cited in the file of this patent 5 UNITED STATES PATENTS 428,084

1,971,945 Winninghofi Aug. 28, 1934 6 Spanner Jan. 14, 1941 Jancke et al Aug. 14, 1942 Overbeek et a1. July 19, 1949 FOREIGN PATENTS Great Britain May 7, 1935 

